In nearly all laser systems, the amount of output power achievable is limited primarily by the amount of available gain as well as the thermal effects on the system. However, it is known in the art of lasers and resonators that a plurality of individual lasers may be coupled together to provide a resultant intensity at a given target point which is greater than a simple sum of the individual lasers, as is discussed in co-pending U.S. patent application, Ser. No. 07/955,810, filed Oct. 2, 1992, entitled "Scalable Laser System Using a Coupled Multiple Output Resonator" now U.S. Pat. No. 5,289,492.
In particular, it is known in the field of high-powered lasers that if N individual uncoupled lasers, each having a single output and sufficient reflected feedback light (called self-feedback) to sustain lasing action, are each focused on a distant target, the peak intensity seen at the target will be equal to N.times.I; where N is the number of lasers and I is the intensity of a single laser. However, if the N lasers are coupled, i.e., phase locked and running at the same frequency or mode, such that the phase between output beams is constant, and they are in-phase with each other at the target, the intensity seen at the target will be N.sup.2 .times.I (i.e., the coherent sum).
However, to maintain phase-lock for multiple cavities, the cavity length of each laser must be kept substantially within an integer multiple of the lasing wavelength of each of the lasers. How closely this requirement must be met depends on the number of lasers and the type of coupling used. However, it is difficult to achieve this level of cavity length matching, due to vibration and thermal effects and because (for high power lasers) it requires precisely controlling very large mirrors.
Another way of achieving N.sup.2 I output intensity is to use a single long laser cavity (not individual lasers), such as a folded cavity, and tap-off output beams from various different locations in the cavity. Such a laser is called a multiple output resonator (or laser) or MOR, as is known.
In the area of optical fiber lasers, the maximum amount of gain available is much less than that available for open air cavities. Furthermore, the aforementioned techniques are not readily applicable in the area of optical fiber lasers. Consequently, to date, the art has not successfully coupled individual fiber lasers to provide a higher power scalable fiber laser system.